OF THE DIATOME.E. 63 



Prof. Smith thus alludes to Mr. Carter's \-iews : — '' The cii'cumstance 

 dwelt upon by Mr. Carter as having an important bearing upon the rationale 

 of the process, viz. that one of the conjugating finistules is invariablj^ smaller 

 than the other, is altogether at variance -^^ith my experience, and is totally 

 irreconcilable with the process as it occurs in the genera mentioned under 

 the third and foui'th classes. 1 am therefore disposed to believe that the 

 difference in size noticed by Mr. Carter was a mere accidental diversity, and 

 of no essential signification." 



The four typical modes of conjugation established by Prof. Smith have 

 their occuiTence thus explained (>Sywops. ii. p. xiii) : — " The functions of 

 life and growth are not suspended dimng the act of conjugation ; and 

 in consequence self-division may take place at any stage of the process 

 which accompanies the formation of the reproductive body, or the latter 

 process may intnide upon, or arrest any step in the progress of self-divi- 

 sion. 



*' In the fii'st mode of conjugation, as occurring in Epithemia, &c., self- 

 division may be regarded as in the earliest stage of its progress, which merely 

 involves the separation of the endochrome of the parent frustules into two 

 portions, but does not include such a differentiation of these portions as 

 renders them capable of the conjugative act : the endochrome capable of 

 conjugating with these segregated portions must be sought for in other 

 frustules ; hence the process in these genera involves the presence of two 

 parent frustules, and results in the production of two sporangia. 



" In the second mode, met with in Himantidium, the progress of separa- 

 tion is arrested at a still earlier stage ; no differentiation has taken place, 

 and conjugation intervening, necessitates the union of the entire contents of 

 two parent frustules to form a single sporangium. 



" In the third mode, the progress of the separation of the endochrome in 

 the parent frustule must be considered as so far advanced that complete 

 differentiation has taken place. In eveiy respect but the formation of new 

 valves, self- division has been completed ; the incomplete fnistules are there- 

 fore prepared for conjugation, which, intervening at this stage, leads the 

 observer to believe that but one fi-ustule has been concerned in the produc- 

 tion of the single sporangium. This we see in Melosira and the other genera 

 mentioned under this class. 



"And lastly, self-division occurring during the progress of conjugation, 

 the endochrome becomes segregated in the veiy act of intermingling, and a 

 single frustule, whose contents have been abeady differentiated, gives rise to 

 two sporangia, as in Achnaiithes and Rhahdonema . 



" Nor is the self-dividing disposition in aU eases permanently arrested by 

 the complete formation of the sporangium. Having assumed the form of the 

 parent frustules, with a great increase in size (the enlargement in dimen- 

 sions being in some cases due to the accumulation of the contents of the two 

 conjugating frustules, and in others to a rapid assimilation of nutritive 

 material from the surrounding medium), the sporangial frustule immediately 

 submits to self- division, and by the repetition of this act developes a series 

 of fnistules equal in size to the original product of the conjugating process. 

 This is notably the case in the filamentous species, as may be easily seen 

 in Melosira, in Orthosira, and in Himantidium. How far this self-division 

 may be carried in the sporangial frustules is at present unknown ; it is pro- 

 bably of short duration, as we rarely meet with any considerable number of 

 frustules characterized by the enlarged size of the sporangial form. In most 

 cases an arrest of growth, and consequently of self- division, seems imme- 

 diately to follow the complete formation of the sporangia, and the reproduc- 



